It is known that seeds, cereals, flours and fertilizers are generally stored in rather high reservoirs known as silos, whose general shape is cylindrical or prismatic. These silos are loaded from the top and emptied at the bottom by the extraction of the ensilaged products, which are then fed into a screw conveyor, a chain conveyor, a belt or pneumatic conveyor or else loaded onto a vehicle in bulk.
Delivering ensilaged products to a conveyor or a vehicle is rather difficult to guarantee, especially in the case of pulverulent products because of their very poor flow characteristics.
The first silos used for this operation had conical-shaped bottoms. The walls converged downwards to an orifice for the discharge of ensilaged products. The advantage of this type of construction is that it facilitates the flow of products to the outlet, which travel along the inclined wall of the bottom cone, are extracted from the silo, fall by gravity to fill a conveyor or load a truck. Nevertheless, there are two disadvantages to this first form of construction: first, over the entire height of the bottom cone of each silo, the loss of useful volume for product storage, as compared to the volume of a flat-bottom silo of the same height and the same section, is equal to twice the interior volume of the bottom cone. Second, it is clear that the silo can not rest on the ground on its conical bottom portion; consequently, is must be provided with a strong bottom support, which unfortunately increases considerably the price of the silo installed at the operation site.
A flat bottom silo was first designed to eliminate the considerable loss in useful volume of the silo due to the conical outlet at the bottom. The products were emptied by means of and along an Archimedean screw, mounted horizontally above the bottom of the silo and on a diameter of the latter; the said extraction screw rotating on its longitudinal axis and simultaneously pivoting around the vertical symmetry axis of the silo.
Because of the double rotation of the Archimedean screw, the ensilaged products are carried to an outlet orifice provided either in the silo bottom or at the base of its vertical lateral wall.
This second construction also has its disadvantage. The Archimedean screw supports the entire mass of products; consequently, the torque needed for its movement has to be considerable, especially for high density, poor flow products (plaster or fertilizer, for example). Because of this fact, the Archimedean screw soon becomes unusable in favorable conditions. To overcome these drawbacks, a flat-bottom silo was then designed, having above its bottom portion a conical deflector, its apex turned toward the top of the silo and its lateral wall connected to a vibrating motor. When this vibrator is turned on, the ensilaged products flow regularly and continuously along the deflector, fall to its base and onto the wall of a conical receiver located under the deflector and equipped with a central opening for the extraction of products.
The first disadvantage of this third design is that the conical receiver at its extreme bottom part constitutes a considerable loss in useful volume for the silo. It is quite true that the height of the receiver is definitely less than that of the cone terminating the bottom part of the silo in the first design, but, nevertheless, the loss in useful volume still exists and this characteristic is always unfortunate, for it must be remembered that the characteristic property of a silo is that it is able to store a maximum volume of products.
The second and far more important disadvantage of this third design has already been discussed in the preceding paragraphs: Since a vibrating deflector placed above a conical receiver is used at the bottom part of the silo, it is obvious that such a silo can not be directly placed on the ground. The same support system as the one mentioned above has to be implemented, increasing, on the one hand, the silo cost, and the support system constituting, on the other hand, another loss in useful volume since it is impossible to store products over the entire overall height of the silo.
As described for example in French Pat. No. 1 165 985, there is also a fourth type of silo construction, in which the emptying apparatus is composed of discharge conveyors in the form of bands or belts.
In this type of construction, the silo is of irregular section, an upper cylinder extending downwards by means of a step in the form of a parallelepipedic foot. The arrangement of this foot, corresponding to a widening of the silo base, is necessary to decompress the ensilaged products in the zone where they are delivered to the conveyors.
The drawback of this fourth construction is obvious. It is particularly costly because, from the architectural point of view, it is much more complicated to design a cylinder resting on a parallelepiped than a cylinder alone, the latter being the easiest shape to construct where silos are concerned.
To overcome this structural difficulty, a further silo design was adopted, entirely cylindrical and flat-bottomed, such as the one described in German patent application No. 2 140 239.
In a construction of this type, in the bottom portion of the silo, above its bottom surface, there is a conical deflector joined to counter-bibs provided on the inner face of the vertical silo wall, level with the deflector. This facilitates the flow of ensilaged products by a kind of baffle arrangement.
This fifth construction still has two major drawbacks which are inherent to the actual design of the baffle arrangement. In the first place, the ensilaged products are progressively compressed while flowing, necessarily causing irregular flow and poor operation of the extraction device. In the second place, the extraction means provided in the fifth device is composed of a moving extractor placed downstream of a fixed horizontal table in the shape of a circular ring, this table being backed against the deflector base or against the vertical wall base. Due to this horizontal table arrangement, it is obvious that a portion of the materials to be extracted will stagnate on the table and form a residual volume ressembling a sloping heap, partly supported on the table and partly backed against the vertical wall of the deflector or silo. There is no way to extract this heap from the silo, unless an additional manual operation is put into effect outside the silo. In other words, the drawback of this fifth construction is that it causes a large portion of the products to stagnate on the fixed table to the point of rotting unless workmen use rakes to extract them.